Hello, I've been working as flash developer for 6 years. I have worked on various projects that was related with videos creation/editing. So I have experience of working with ffmpeg library.
The Save/load feature can be implemented using XML format. In this case it would contain paths to a png image, a mp3 sound, a swf template. The file will have small size but to open it to another PC/MAC user will need to copy all files that specified in the xml (the image, the sound, the template). To avoid the user to do this I can embed all this files inside the file format using the FZip library. In this case the file size will be bigger but the user can copy and paste it to any directory or computer and it will work. Also, the file extension will be associated with your application. So when user double click on the file, it will be opened in your application.
The templates should be made as swf to use in flash. It's also Ok if they will be done as mp4 videos, I think.
As Adobe AIR and ffmpeg works in PC and MAC there shouldn't be problem to run your application on the both operation systems.
The process of development will be divided on two main stages.
1. Creating the GUI.
2. Implementing features.
For developing I use Adobe Flash.